The present study investigated the effect of the number of spectral components on the threshold in quiet of infrasound stimuli. Stimuli with one, two, or three sinusoidal components were presented monaurally to the ear with a low-distortion infrasound reproduction system. All components of the complex tones had the same level relative to their respective pure-tone threshold. The data are consistent with the assumption of a (perceptually weighted) intensity integration at threshold: The level at threshold for each component is 3 dB lower when two components were presented simultaneously. Thresholds decrease further, when a three-tone complex is used.

Several studies showed that the human auditory system is sensitive to infrasound. The present study investigated if non-linear processes within the ear generate distortion products, which may explain the auditory sensitivity to infrasound. Pure-tone infrasound stimuli were presented to the ears of 16 listeners with a low-distortion sound reproduction system via an ear insert. Simultaneously, the sound in the ear canal was recorded with a high-sensitivity probe microphone. In addition, detection thresholds in quiet for these stimuli were determined in all listeners. All of the listeners showed distortion products, which were detected by analyzing the sound recorded in their ear canals during infrasound stimulation, for at least one of the signal frequencies. The sound pressure levels of the distortion products were well below the reference thresholds at the corresponding frequencies. For each signal frequency, the listeners’ detection thresholds in cases with ear-generated distortion products did not differ significantly from those in cases without. Thus, the present data do not support the hypothesis that distortion products play a major role in the auditory perception of infrasound.

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